Compatibility of cardiac pacemakers, implantable defibrillators and other types of active implantable medical devices with magnetic resonance imaging (MRI) and other types of hospital diagnostic equipment has become a major issue. If one goes to the websites of the major cardiac pacemaker manufacturers in the United States, which include St. Jude Medical, Medtronic and Boston Scientific (formerly Guidant), one will see that the use of MRI is generally contra-indicated with pacemakers and implantable defibrillators. See also:                (1) “Safety Aspects of Cardiac Pacemakers in Magnetic Resonance Imaging”, a dissertation submitted to the Swiss Federal Institute of Technology Zurich presented by Roger Christoph Luchinger, Zurich 2002;        (2) “I. Dielectric Properties of Biological Tissues: Literature Survey”, by C. Gabriel, S. Gabriel and E. Cortout;        (3) “II. Dielectric Properties of Biological Tissues: Measurements and the Frequency Range 0 Hz to 20 GHz”, by S. Gabriel, R. W. Lau and C. Gabriel;        (4) “III. Dielectric Properties of Biological Tissues: Parametric Models for the Dielectric Spectrum of Tissues”, by S. Gabriel, R. W. Lau and C. Gabriel; and        (5) “Advanced Engineering Electromagnetics, C. A. Balanis, Wiley, 1989;        (6) Systems and Methods for Magnetic-Resonance-Guided Interventional Procedures, U.S. Pub. No. 2003/0050557, now U.S. Pat. No. 7,844,319 to Susil et al.;        (7) Multifunctional Interventional Devices for MRI: A Combined Electrophysiology/MRI Catheter, by, Robert C. Susil, Henry R. Halperin, Christopher. Yeung, Albert C. Lardo and Ergin Atalar, MRI in Medicine, 2002; and        (8) Multifunctional Interventional Devices for Use in MRI, U.S. patent application Ser. No. 60/283,725, filed Apr. 13, 2001.        (9) Characterization of the Relationship Between MR-Induced Distal Tip Heating in Cardiac Pacing Leads and the Electrical Performance of Novel Filtered Tip Assemblies, by Robert S. Johnson, Holly Moschiano, Robert Stevenson, Scott Brainard, Sam Ye, Joseph E. Spaulding, Warren Dabney, 17th Scientific Meeting & Exhibition of the International Society for Magnetic Resonance in Medicine, Honolulu, Hi., 18-24 Apr. 2009, Page No. 307.        (10) Comparative Analyses of MRI-Induced Distal Heating and Novel Filtered Cardiac Pacing Leads Using Two Geometric Configurations, by F. G. Shellock, Holly Moschiano, Robert Johnson, Robert Stevenson, Scott Brainard, Sam Ye and Warren Dabney, 17th Scientific Meeting & Exhibition of the International Society for Magnetic Resonance in Medicine, Honolulu, Hawaii, 18-24 Apr. 2009, Page No. 3104.        
The contents of the foregoing are all incorporated herein by reference.
However, an extensive review of the literature indicates that MRI is indeed often used with pacemaker, neurostimulator and other active medical device (AMD) patients. The safety and feasibility of MRI for patients with cardiac pacemakers is an issue of gaining significance. The effects of MRI on patients' pacemaker systems have only been analyzed retrospectively in some case reports. MRI is one of medicine's most valuable diagnostic tools. MRI is, of course, extensively used for imaging, but is also used for interventional medicine (surgery). In addition, MRI is used in real time to guide ablation catheters, neurostimulator tips, deep brain probes and the like. An absolute contra-indication for pacemaker patients means that pacemaker and ICD wearers are excluded from MRI. This is particularly true of scans of the thorax and abdominal areas. Because of MRI's incredible value as a diagnostic tool for imaging organs and other body tissues, many physicians simply take the risk and perform MRI on a patient with an implanted pulse generator (IPG). The literature indicates a number of precautions that physicians should take in this case, including limiting the power of the MRI RF Pulsed field (Specific Absorption Rate—SAR level), programming the IPG to fixed or asynchronous pacing mode, and then careful reprogramming and evaluation of the device and patient after the procedure is complete. There have been reports of latent problems with cardiac pacemakers or other AMDs after an MRI procedure, sometimes occurring many days later. Moreover, there are a number of recent papers that indicate that the SAR level is not entirely predictive of the heating that would be found in implanted lead wires or devices. For example, for magnetic resonance imaging devices operating at the same magnetic field strength and also at the same SAR level, considerable variations have been found relative to heating of implanted lead wires. It is speculated that SAR level alone is not a good predictor of whether or not an implanted device or its associated lead wire system will overheat.
There are three types of electromagnetic fields used in an MRI unit. The first type is the main static magnetic field designated B0 which is used to align protons in body tissue. The field strength varies from 0.5 to 3.0 Tesla in most of the currently available MRI units in clinical use. At the International Society for Magnetic Resonance in Medicine (ISMRM), which was held on 5 and 6 Nov. 2005, it was reported that field strengths of certain research MRI systems are increasing to levels as high as 11.7 Tesla. This is over 100,000 times the magnetic field strength of the earth. A static magnetic field can induce powerful mechanical forces and torque on any magnetic materials implanted within the patient. This would include certain components within an implanted device such as the cardiac pacemaker itself and or lead wire systems. It is not likely (other than sudden system shut down) that the static MRI magnetic field can induce currents into the pacemaker lead wire system and hence into the pacemaker itself. It is a basic principle of physics that a magnetic field must either be time-varying as it cuts across the conductor, or the conductor itself must move within the magnetic field for currents to be induced.
The second type of field produced by magnetic resonance imaging is the pulsed RF field which is generated by the body coil or head coil. This is used to change the energy state of the protons and elicit MRI signals from tissue. The RF field is homogeneous in the central region and has two main components: (1) the magnetic field is circularly polarized in the actual plane; and (2) the electric field is related to the magnetic field by Maxwell's equations. In general, the RF field is switched on and off during measurements and usually has a frequency of 21 MHz to as much as 500 MHz, depending upon the static magnetic field strength. The frequency of the RF pulse varies with the field strength of the main static field where, for a hydrogen MRI scanner: RF PULSED FREQUENCY in MHz=(42.56)×(STATIC FIELD STRENGTH IN TESLA).
The third type of electromagnetic field is the time-varying magnetic gradient fields designated Gx, Gy, Gz, which are used for spatial localization. These change their strength along different orientations and operating frequencies on the order of 2-5 kHz. The vectors of the magnetic field gradients in the x, y and z directions are produced by three sets of orthogonally positioned coils and are switched on only during the measurements. In some cases, the gradient field has been shown to elevate natural heart rhythms (heart beat). This is not completely understood, but it is a repeatable phenomenon.
It is instructive to note how voltages and EMI are induced into an implanted lead wire system. At very low frequency (VLF), voltages are induced at the input to the cardiac pacemaker as currents circulate throughout the patient's body and create voltage drops. Because of the vector displacement between the pacemaker housing and, for example, the Tip electrode, voltage drop across the resistance of body tissues may be sensed due to Ohms Law and the circulating current of the RF signal. At higher frequencies, the implanted lead wire systems actually act as antennas where currents are induced along their length. These antennas are not very efficient due to the damping effects of body tissue; however, this can often be offset by extremely high power fields (such as MRI RF pulsed fields) and/or body resonances.
At the frequencies of interest in MRI, RF energy can be absorbed and converted to heat. The power deposited by RF pulses during MRI is complex and is dependent upon the power (Specific Absorption Rate (SAR) Level) and duration of the RF pulse, the transmitted frequency, the number of RF pulses applied per unit time, and the type of configuration of the RF transmitter coil used. The amount of heating also depends upon the volume of tissue imaged, the electrical resistivity of the tissue, and the configuration of the anatomical region imaged. There are also a number of other variables that depend on the placement in the human body of the Active Implantable Medical Device (AIMD) and its associated lead wire(s). For example, the routing of the lead (right pectoral vs left pectoral, abdominal) and the lead length are also very critical as to the amount of induced current and heating that would occur. Also, distal Tip electrode design is very important as the distal Tip electrode itself can act as its own antenna wherein eddy currents can create heating. The cause of heating in an MRI environment is twofold: (a) RF field coupling to the lead can occur which induces significant local heating; and (b) currents induced between the distal Tip electrode and tissue during MRI RF pulse transmission sequences can cause local Ohms Law heating in tissue next to the distal Tip electrode of the implanted lead. The RF field of an MRI scanner can produce enough energy to induce lead wire currents sufficient to destroy some of the adjacent myocardial tissue. Tissue ablation has also been observed. The effects of this heating are not readily detectable by monitoring during the MRI. Indications that heating has occurred would include an increase in pacing threshold or loss of capture, venous ablation, Larynx or esophageal ablation, myocardial perforation and lead penetration, or even arrhythmias caused by scar tissue. Such long term heating effects of MRI have not been well studied yet for all types of AIMD lead wire geometries. There can also be localized heating problems associated with various types of electrodes in addition to Tip electrodes. This includes Ring electrodes or Pad electrodes. Ring electrodes are commonly used with a wide variety of implanted devices including cardiac pacemakers, neurostimulators, probes, catheters and the like. Pad electrodes are very common in neurostimulator applications. For example, spinal cord stimulators or deep brain stimulators can include a plurality of Pad electrodes to make contact with nerve tissue. A good example of this also occurs in a cochlear implant. In a typical cochlear implant there would be approximately 20 Ring electrodes that the physician places by pushing the electrode into the cochlea. Several of these Ring electrodes make contact with auditory nerves.
There are additional problems possible with implantable cardioverter defibrillators (ICDs). The programmable sensitivity in ICD biological sense circuits is normally much higher (more sensitive) than it is for pacemakers; therefore, ICDs may falsely detect a ventricular tachyarrhythmia and inappropriately deliver therapy. In this case, therapy might include anti-tachycardia pacing, cardio version or defibrillation (high voltage shock) therapies. MRI magnetic fields may prevent detection of a dangerous ventricular arrhythmia or fibrillation. There can also be heating problems of ICD leads which are expected to be comparable to those of pacemaker leads. Ablation of vascular walls is another concern. Finally, in the presence of the main static field of the MRI the ferromagnetic core of this transformer tends to saturate thereby preventing the high voltage capacitor from fully charging up. This makes it highly unlikely that an ICD patient undergoing an MRI would receive an inappropriate high voltage shock therapy. While ICDs cannot charge during MRI due to the saturation of their ferro-magnetic transformers, the battery will be effectively shorted and lose life. This is a highly undesirable condition.
In summary, there are a number of studies that have shown that MRI patients with active medical devices, such as cardiac pacemakers, can be at risk for potential hazardous effects. However, there are a number of reports in the literature that MRI can be safe for imaging of pacemaker patients when a number of precautions are taken (only when an MRI is thought to be an absolute diagnostic necessity). These anecdotal reports are of interest, however, they are certainly not scientifically convincing that all MRI can be safe. As previously mentioned, just variations in the pacemaker lead wire length can significantly affect how much heat is generated. From the layman's point of view, this can be easily explained by observing the typical length of the antenna on a cellular telephone compared to the vertical rod antenna more common on older automobiles. The relatively short antenna on the cell phone is designed to efficiently couple with the very high frequency wavelengths (approximately 950 MHz) of cellular telephone signals. In a typical AM and FM radio in an automobile, these wavelength signals would not efficiently couple to the relatively short antenna of a cell phone. This is why the antenna on the automobile is relatively longer. An analogous situation exists with an AMD patient in an MRI system. If one assumes, for example, a 3.0 Tesla MRI system, which would have an RF pulsed frequency of 128 MHz, there are certain implanted lead lengths that would couple efficiently as fractions of the 128 MHz wavelength. It is typical that a hospital will maintain an inventory of various leads and that the implanting physician will make a selection depending on the size of the patient, implant location and other factors. Accordingly, the implanted or effective lead wire length can vary considerably. There are certain implanted lead wire lengths that do not couple efficiently with the MRI frequency and there are others that would couple very efficiently and thereby produce the worst case for heating.
The effect of an MRI system on the function of pacemakers, ICDs and neurostimulators depends on various factors, including the strength of the static magnetic field, the pulse sequence (gradient and RF field used), the anatomic region being imaged, lead length and trajectory, and many other factors. Further complicating this is the fact that each patient's condition and physiology are different and each manufacturer's IPGs behave differently. Most experts still conclude that MRI for the pacemaker patient should not be considered safe. Paradoxically, this also does not mean that the patient should not receive MRI. The physician must make an evaluation given the pacemaker patient's condition and weigh the potential risks of MRI against the benefits of this powerful diagnostic tool. As MRI technology progresses, including higher field gradient changes over time applied to thinner tissue slices at more rapid imagery, the situation will continue to evolve and become more complex. An example of this paradox is a pacemaker patient who is suspected to have a cancer of the lung. RF ablation treatment of such a tumor may require stereotactic imaging only made possible through real time fine focus MRI. With the patient's life literally at risk, the physician with patient informed consent may make the decision to perform MRI in spite of all of the previously described attendant risks to the pacemaker system.
It is clear that MRI will continue to be used in patients with both external and active implantable medical devices. There are a number of other hospital procedures, including electrocautery surgery, lithotripsy, etc., to which a pacemaker patient may also be exposed. Accordingly, there is a need for AMD system and/or circuit protection devices which will improve the immunity of active medical device systems to diagnostic procedures such as MRI.
As one can see, many of the undesirable effects in an implanted lead system from MRI and other medical diagnostic procedures are related to undesirable induced currents in the conductor(s) of the lead system and/or its distal Ring electrode (or Ring). This can lead to overheating either in the lead or at the body tissue at the distal Ring electrode. For a pacemaker application, these currents can also directly stimulate the heart into sometimes dangerous arrhythmias.
MRI scanners that are commercially used in the market have evolved generally from 0.5 T to 1.5 T and higher. The most common systems in use today are 1.5 T and 3 T which have pulsed RF frequencies of 64 MHz and 128 MHz respectively. There are also a number of research machines that are evolving at 5 T, 7 T, 11 T and even higher. These would, of course, have higher RF pulse frequencies in accordance with the Lamour equation. For hydrogen scanners, the Lamour frequency which is equal to the MRI RF pulsed frequency is equal to 42.56 times the MRI scanner static magnetic field strength. For example for a 1.5 Tesla scanner, the RF field strength is 63.84 MHz. There are other types of scanners such as phosphorous scanners which would have different Lamour frequencies. There are a number of reasons why an active medical device patient may require MRI. These can range from neurologic disorders to cardiac disorders or to any type of soft tissue injury. 1.5 Tesla systems are fine for certain types of imaging, however, 3 T and higher can provide significantly improved imaging to diagnose a certain class of disorders. Accordingly, it is difficult to predict to what static magnetic field strength scanner an AIMD patient may eventually be exposed. In fact, over a patient's lifetime they may even have a need at one time to be scanned at 1.5 T and a later need to be scanned at 3 T.
There is a need for a composite RF current attenuator which can be placed at one or more locations along the active implantable medical device lead system, which presents a high impedance that prevents RF currents from circulating at selected frequencies of the MRI system. Preferably, such novel broadband MRI filters would be designed to provide a high impedance at 64 MHz (1.5 T), 128 MHz (3 T) and also at higher frequencies. The present invention fulfills these needs and provides other related advantages.